The lesson is that regular humans like us can behave monstrously. Every time we deny their humanity, we ignore the fact that this is a human problem, not just a matter of a few random inhuman monsters. If we can recognize that they're not special, they're people like us, we can better guard against repeating what enables and encourages people to go this way.

We might not have our current problems with the rise of fascism if we hadn't spent decades telling ourselves that the Nazis were unusual monsters, and gradually forgetting the reality that they were ordinary people just like us.

Exactly opposite of what I mean. But you do you.